I have never much warmed to Enid Bagnold's play: what others call its high comic style sounds to me arch, precious and exhibitionist. But, if this 1956 Haymarket hit is to be revived, I cannot imagine a snappier or more sensitively acted version than that by Michael Grandage.

Bagnold's plot revolves around a mysterious stranger, Miss Madrigal, employed by a high-born military widow, Mrs St Maugham, as governess to her self-dramatising granddaughter, Laurel. The outer action concerns the gradual revelation (confirmed when a judge comes to lunch) that Miss Madrigal once stood trial for murder.

But the inner action deals with the governess's attempt to rescue her teenage charge from her grandmother: the lying Laurel can no more flourish in this world of licensed eccentricity, than rhododendrons can prosper in Mrs St Maugham's arid chalk garden.

Bagnold's play strenuously tries to have it both ways. It implicitly condemns a world of post-Edwardian snobbery; yet it delights in Mrs St Maugham's sub-Wildean epigrams such as "power and privilege make selfish people but gay ones". By a miracle of acting, Margaret Tyzack manages to reconcile one to this arthritic ogre - by making it clear that she is trapped inside a self-created persona. Tyzack sails through the play like a stately warship firing on all and sundry. But under the tart, dismissive putdowns - such as asking her daughter "How can you wear beige, with your skin that colour?" - Tyzack shows that Mrs St Maugham is a sad, solitary relic as much in need of rescuing as her grand-daughter.

Grandage's discovery of a sub-textual emotional truth is confirmed by Penelope Wilton's equally mesmerising performance as Miss Madrigal. First seen silently sitting in a darkened conservatory, Wilton presents us with a woman wreathed in loneliness who has acquired an inner strength through her survival of an unjust murder-charge. Far from being mousy or self-deprecating, Wilton robustly defies her employer, and is astonishing in a scene where Laurel seeks to probe her guilty past: smiling serenely, she tells her teenage inquisitor "you take my breath away" in a tone of impenetrable irony. Wilton, in short, plays Miss Madrigal as a tough cookie who, through her self-awareness becomes a positive force for good.

Clifford Rose lends the supposedly omnipotent judge a dithering vulnerability, and Jamie Glover invests a crime-obsessed manservant with a fine neurotic flakiness. But this is typical of a production that takes a play once dubbed "the last drawing-room comedy", and discovers, beneath its ostentatious phrase-making, an unexpected humanity.